How to set an alert in Android to fire at given date and time?

This example demonstrates how to do I an alert in Android to fire at a given date and time in android.

Step 1 − Create a new project in Android Studio, go to File ⇒ New Project and fill all required details to create a new project.

Step 2 − Add the following code to res/layout/activity_main.xml.

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
   xmlns:tools="http://schemas.android.com/tools"
   android:layout_width="match_parent"
   android:layout_height="match_parent"
   android:orientation="vertical"
   tools:context=".MainActivity">
</LinearLayout>

Step 3 − Add the following code to src/MainActivity.java

package app.com.sample;
import androidx.appcompat.app.AppCompatActivity;
import androidx.fragment.app.DialogFragment;
import android.app.AlarmManager;
import android.app.PendingIntent;
import android.app.TimePickerDialog;
import android.content.Context;
import android.content.Intent;
import android.os.Bundle;
import android.widget.TimePicker;
import java.text.DateFormat;
import java.util.Calendar;
import java.util.Objects;
public class MainActivity extends AppCompatActivity implements TimePickerDialog.OnTimeSetListener{
   String timeText;
   @Override
   protected void onCreate(Bundle savedInstanceState) {
      super.onCreate(savedInstanceState);
      setContentView(R.layout.activity_main);
      DialogFragment timePicker = new TimePickerFragment();
      timePicker.show(getSupportFragmentManager(), "time picker");
   }
   @Override
   public void onTimeSet(TimePicker view, int hourOfDay, int minute) {
      Calendar c = Calendar.getInstance();
      c.set(Calendar.HOUR_OF_DAY, hourOfDay);
      c.set(Calendar.MINUTE, minute);
      c.set(Calendar.SECOND, 0);
      updateTimeText(c);
      startAlarm(c);
   }
   private void updateTimeText(Calendar c) {
      timeText = "Alarm set for: ";
      timeText += DateFormat.getTimeInstance(DateFormat.SHORT).format(c.getTime());
   }
   private void startAlarm(Calendar c) {
      AlarmManager alarmManager = (AlarmManager) getSystemService(Context.ALARM_SERVICE);
      Intent intent = new Intent(this, NotificationPublisher.class);
      PendingIntent pendingIntent = PendingIntent.getBroadcast(this, 1, intent, 0);
      if (c.before(Calendar.getInstance())) {
         c.add(Calendar.DATE, 1);
      }
      Objects.requireNonNull(alarmManager).setExact(AlarmManager.RTC_WAKEUP,
      c.getTimeInMillis(), pendingIntent);
   }
}

Step 4 − Create a new class NotificationPublisher and add the following code

package app.com.sample;
import android.app.Notification;
import android.content.BroadcastReceiver;
import android.content.Context;
import android.content.Intent;
import androidx.core.app.NotificationCompat;
public class NotificationPublisher extends BroadcastReceiver {
   @Override
   public void onReceive(Context context, Intent intent) {
      NotificationHelper notificationHelper = new NotificationHelper(context);
      NotificationCompat.Builder nb = notificationHelper.getChannelNotification();
      notificationHelper.getManager().notify(1, nb.build());
      Notification notification = nb.build();
      notification.defaults |= Notification.DEFAULT_VIBRATE;
      notification.defaults |= Notification.DEFAULT_SOUND;
   }
}

Step 5 − Create a new class NotificaitonHelper and add the following code

package app.com.sample;
import android.annotation.TargetApi;
import android.app.NotificationChannel;
import android.app.NotificationManager;
import android.content.Context;
import android.content.ContextWrapper;
import android.os.Build;
import androidx.core.app.NotificationCompat;
class NotificationHelper extends ContextWrapper {
   public static final String channelID = "channelID";
   public static final String channelName = "Channel Name";
   private NotificationManager notificationManager;
   public NotificationHelper(Context base) {
      super(base);
      if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.O) {
         createChannel();
      }
   }
   @TargetApi(Build.VERSION_CODES.O)
   private void createChannel() {
      NotificationChannel channel = new NotificationChannel(channelID, channelName,
      NotificationManager.IMPORTANCE_HIGH);
      getManager().createNotificationChannel(channel);
   }
   public NotificationManager getManager() {
      if (notificationManager == null) {
         notificationManager = (NotificationManager)
         getSystemService(Context.NOTIFICATION_SERVICE);
      }
      return notificationManager;
   }
   public NotificationCompat.Builder getChannelNotification() {
      return new NotificationCompat.Builder(getApplicationContext(), channelID)
         .setContentTitle("Scheduled Alert")
         .setContentText("Your Alert is Ringing")
         .setSmallIcon(R.drawable.ic_alarm);
      }
   }

Step 6 − Create a new class TimePickerFragment and add the following code

package app.com.sample;
import android.app.Dialog;
import android.app.TimePickerDialog;
import android.os.Bundle;
import android.text.format.DateFormat;
import java.util.Calendar;
import androidx.annotation.NonNull;
import androidx.fragment.app.DialogFragment;
public class TimePickerFragment extends DialogFragment {
   @NonNull
   @Override
   public Dialog onCreateDialog(Bundle savedInstanceState) {
      Calendar c = Calendar.getInstance();
      int hour = c.get(Calendar.HOUR_OF_DAY);
      int minute = c.get(Calendar.MINUTE);
      return new TimePickerDialog(getActivity(), (TimePickerDialog.OnTimeSetListener) getActivity(),
      hour, minute, DateFormat.is24HourFormat(getActivity()));
   }
}

Step 7 − Add the following code to androidManifest.xml

<?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="http://schemas.android.com/apk/res/android"
   package="app.com.sample">
   <application
      android:allowBackup="true"
      android:icon="@mipmap/ic_launcher"
      android:label="@string/app_name"
      android:roundIcon="@mipmap/ic_launcher_round"
      android:supportsRtl="true"
      android:theme="@style/AppTheme">
      <activity android:name=".MainActivity">
         <intent-filter>
            <action android:name="android.intent.action.MAIN" />
            <category android:name="android.intent.category.LAUNCHER" />
         </intent-filter>
      </activity>
   <receiver android:name=".NotificationPublisher"></receiver>
</application>

Let's try to run your application. I assume you have connected your actual Android Mobile device with your computer. To run the app from the android studio, open one of your project's activity files and click the Run  icon from the toolbar. Select your mobile device as an option and then check your mobile device which will display your default screen −